Accurate high quality reproduction of audio signals, also referred to as high fidelity reproduction, is among others preferred in the music industry while monitoring sound during for instance music recording, sound mastering and audio engineering. Accurate high quality reproduction of pre-recorded audio signals is also preferred by musicians and audiophiles.
A loudspeaker is a device that converts an electrical audio signal or impulse into corresponding sound. The loudspeaker typically consists of a purpose-engineered enclosure, housing at least one loudspeaker driver, also called transducer, and associated electronic equipment, such as crossover circuits and amplifiers. A transducer is a device that converts an electrical signal into variations in a physical quantity, such as sound, the conversion being the operating principle behind generally available loudspeaker systems and applications.
Loudspeaker enclosures range in design from simple, rectangular particle-board chambers to highly sophisticated cabinets with advanced geometries. Those advanced, more complex enclosures may incorporate composite materials and state of the art components. The geometry of the enclosures, internal and external may also comprise internal sub-chambers defined and delimited by passive acoustic radiators and/or passages there between. Expressions used for such inner or outer passages between sub-chambers are vents (ventilation passages), ports and diaphragms.
Conventional loudspeakers use the mentioned variety of enclosures, chambers and sub-chambers, components and driver arrangements to reproduce sound within and beyond the commonly accepted range of human hearing, which is between 20 Hz and 20 kHz. A full-range driver is a type of driver designed to reproduce most of the audible frequencies. Due to physical and technical limitations no driver in use today can audibly reproduce all the frequencies within this range all by itself. To overcome this limitation, it is therefore common for loudspeakers to utilise at least two distinct drivers. The low- and mid-range frequencies from 20 Hz to up to 1500 Hz are produced using a woofer, and the high range frequencies from 1500 Hz to 20 kHz are produced using a tweeter respectively.
Despite use of a twin driver arrangement according to the above, very low frequencies are usually still attenuated well below audible levels. To counteract this, the so-called high-end loudspeakers typically reproduce sound utilising at least three drivers, using a dedicated low frequency woofer for reproducing the low frequencies (bass), and the mid-range driver and the tweeter for reproducing the remaining frequencies respectively. Dedicated sub-woofers also employ this low frequency woofer to the same effect.
When more than one driver is used in a loudspeaker, a crossover circuit is required to ensure that the multiple drivers do not reproduce the same frequencies, which could result in interference, such as undesired coloration or cancellation of the sound waves being generated. The crossover circuit in most cases is a combination of simple band-pass filters constructed using inductors, capacitors and resistors of high quality. The use of a crossover circuit can allow for near optimal sound reproduction, but not without the overhead of lowered operating efficiency and energy loss through dissipated heat. For crossover circuits to function properly and not impair a near optimal reproduction of sound, they need to be made from high quality components. With this requirement comes the inevitable disadvantage of adding considerable hardware costs to a sound system besides increasing complexity of the system.
Yet another difficulty to consider during construction and production of loudspeakers with multiple drivers is tolerance. Precise alignment and positioning of each driver on the baffle of the loudspeaker enclosure with respect to any other drivers is crucial for accurate sound reproduction, as even minor deviations from the optimal alignment can lead to undesired coloration or distortion of the generated sound.
A loudspeaker enclosure can be used as a means to extend the low frequency response of the woofer driver. For example, the enclosure can be designed to resonate at certain low frequencies by ventilating the volume of the air inside the enclosure via a port, thus increasing the low frequency (bass) output from the loudspeaker. Variations of this port, which in effect has the function of a so-called Helmholtz resonator, have been devised to optimise a wide range of woofer drivers. Such enclosures usually require a relatively large internal volume compared with the available space for housing such a volume and the desire to keep outer dimensions of loudspeaker enclosures smallest possible.
Some loudspeakers utilise one or more passive acoustic diaphragms, which is a type of passive acoustic radiator and which will be referred to as passive radiator henceforth, in place of ports. A passive radiator is a driver without a magnet, voice coil and terminal assembly, and is hence not physically connected or wired to the amplifier. When coupled with a suitable driver, the passive radiator vibrates in response to the changing air pressure inside the loudspeaker enclosure caused by the vibrating driver. Unlike for a port, the resonance frequency of the passive radiator can be accurately tuned by changing its vibrating mass. Thus, tuning adjustments for a passive radiator can be accomplished more quickly than for the case of the more conventional bass reflex design, since such corrections can be as simple as a mass adjustment to its diaphragm. Disadvantages are that a passive radiator must be manufactured with small tolerances quite like a driver. This increases production costs besides the limitations in excursion, which applies to passive acoustic diaphragms.
Several types of loudspeaker enclosure designs have been proposed for accurate reproduction of audio signals. One such design uses woofer drivers mounted inside sealed enclosures, with or without additional passive radiators. This type of enclosure provides an excellent transient response characteristic. Nevertheless, this design does not extend the low frequency response of the driver below its own resonance frequency, or below the resonance frequency of the passive radiators if any. Another design, typically created for extending the low frequency response several loudspeakers, utilises a band-pass enclosure design which is achieved by sub-dividing the internal volume of the enclosure into multiple sub-chambers of varying volumes. Many high end sub-woofers on the market use the band-pass enclosure design.
An example of such a design is reflected in U.S. Pat. No. 6,389,146, wherein a band-pass loudspeaker enclosure includes three sub-chambers, the first one being a non-Helmholtz-reflex chamber of a sealed acoustic suspension type, and the remaining two chambers utilising two passive acoustic radiators to achieve two Helmholtz-reflex ventilation tunings. Moreover, multiple of low pass acoustic filters are arranged to provide an acoustic band-pass with a substantially second order high pass characteristic combined with an extended, steeper, at least fourth order slope low pass stop band characteristic. The use of multiple low pass, acoustic filter characteristic filters out internal resonances and minimises their acoustical output. A disadvantage of the described loudspeaker design is that band-pass enclosures tend to have a poorer transient response as compared to that of sealed enclosures.
To achieve optimal low frequency response, band-pass enclosures require considerable internal volume and hence have to be large in size, which means that they become heavy and bulky to handle. Furthermore, both of the aforementioned designs require the use of multiple drivers, a woofer, a mid-range driver and a tweeter, as well as and a crossover circuit for accurate reproduction of audio signals. This not only increases the unit cost, but also leads to coloration of the sound being reproduced. Furthermore, the use of multiple drivers and a crossover circuit also increases energy consumption.
The invention aims at overcoming some of the mentioned disadvantages related to accurate sound reproduction in conventional audio systems generally and loudspeakers and their enclosures in particular.